Ultrafast laser pulses have been shaped so as to alter the relative amounts of the various frequency components or colors of light therein. The best previously known apparatus for doing this includes means for dispersing the frequency components of incident laser pulses that are directed along an input path, passing these components through a collimator, positioning a multielement liquid crystal modulator, LCM, in the path of the light emerging from the collimator, collecting the collimated frequency components after they have passed through the LCM, and directing pulses from the focused frequency components along an output path that is collinear with the input path. Each of the liquid crystal elements is in a different position of the spectrum of dispersed light so that it can control the amount of light in that position. In this apparatus, a first diffraction grating is used to spatially disperse the frequency components of the incident pulses, and a collimating lens is placed between the first diffraction grating and the multielement LCM. A lens focusses the collimated light emerging from the LCM onto a second grating that directs emergent laser pulses having a different shape than the incident pulses along an output path.
Among the disadvantages of the pulse shaper or modulator just described are the difficulty of modulator alignment, the existence of pixel gaps between the elements of the liquid crystal, the need for pixel calibration and on/off isolation of pixels. As a result, the apparatus is restricted to discrete approximations of the desired spectrum.
As an illustration, consider a simple linearly frequency-swept (chirped) pulse. The Fourier transform of such a pulse has phase shifts proportional to .omega..sup.2, so even a moderate sweep will require large phase shifts. Of course, the phase need only be reproduced modulo 2.pi. at each pixel, so the control voltage need not be larger than the full-wave voltage of the modulator. However, LCM's generate a discrete and discontinuous approximation to a parabolic function, which will preclude complex frequency modulation, unless the phase increment between adjacent pixels is less than 2.pi.. This severely restricts the amount of chirp that can be imposed. In fact, some of the interesting potential applications of shaped pulses require much more sophisticated frequency modulation (for example, smoothly sweeping to resonance with a bright state and then holding the frequency and amplitude fixed), and here the limitations will be even more severe.
The uses of such a modulator are limited because of the inherently slow response of liquid crystals to an applied stimulation. Furthermore, each crystal element has to be separately modulated, thereby requiring a parallel drive, and independent amplitude and phase modulation cannot be achieved with one modulator.